The present invention relates to a method of continuously forming three-dimensional parts by heating and stamping using sheets of a high-performance fiber-reinforced composite material which employs as a matrix a thermoplastic resin and in which the reinforcing fiber is oriented, and an apparatus therefor.
The conventional methods of forming resin composite materials are classified by the contents of the reinforcing material and type of resin as shown in FIG. 1.
More specifically, to form a composite material of a thermoplastic resin and a reinforcing material which is 50% or more of the contents by volume, prepreg molding or pultrusion is used. When the contents of the reinforcing material are 50% or less by volume, injection molding or stampable sheet forming is employed. When a composite material is composed of a thermoset resin and a reinforcing material which is 50% or more of the contents by volume, filament winding, pultrusion or hand lay-up molding is used. When the contents of the reinforcing material are 50% or less, SMC, BMC or spraying is used. In FIG. 1, the numerals in parentheses are percentages representing typical contents [See TEXXES Features and Application of Hybrid Fabric By Hiroichi Inoguchi, Industrial Material Vol. 37, No. 1981.1].
FIG. 2 shows the strength of the parts manufactured by various forming methods. The numerals in parentheses are estimated values [see Technological Trends on Stampable Sheets by Suguru Koshimoto, Plasticity and Processing, Vol 29, No. 333 (1988-10) and TEXXES Features and Application of Hybrid Fabric, Hiroichi Inoguchi, Industrial Material, Vol. 37, No. 1 (1989.1)].
The present invention is directed to forming high-strength parts from a composite material which employs as a matrix a thermoplastic resin used in the prepreg forming and pultrusion shown in FIG. 1, and in which reinforcing fiber is oriented.
In both the aforementioned high-strength parts forming method and the conventional fiber reinforced plastic forming methods, stamping is used as one method of forming a resin type composite material. This stamping method is commonly used to produce mass-market consumer products. The stampable sheet forming and SMC shown in FIG. 1 are examples of this stamping method.
Stampable sheet stamping consists of the steps of preheating a blank of a sufficient length and of placing the blank into a mold to form it under pressure, the mold being heated to a temperature at which the resin sets or below.
In this forming method, flow of the resin preheated to its melting point or above is utilized to form the sheet. With some types of resins, the thickness of the parts can be changed by changing the number of preheated sheets piled on one another.
This conventional stamping method employs a material in which a material, a thermoset or thermoplastic resin, is reinforced by glass or carbon fiber (short fiber, long fiber or long fiber mat), and utilizes flow of the material which occurs within the closed mold during forming. To ensure excellent flow, a material whose fiber contents are 50% or less is generally used.
The reinforcing fiber conventionally used in the resin type composite materials formed by stamping is either short or long chopped strands or chopped or continuous strand mat. Since such fibers can be relatively freely deformed in the base matrix during forming, they are suited to stamping.
On the other hand, the present invention is aimed at a method of stamping sheets of a high-performance fiber reinforced composite material (one type of so-called advanced composites) which employs as a matrix a thermoplastic resin and in which the reinforcing fiber is oriented. Fibers (cloths) having various orientations, typical examples of which are shown in FIG. 3, can be used as the reinforcing fiber in the high-performance fiber reinforced composite material (See Newest Composite Technological List, Newest Composite Technological Survey Editing Committee, published in March 1990 by Sangyo Gijutsu Service Center). Reinforcing fiber of a volume of 50% or more may be achieved. Also, since the strength of the fiber, stronger than a resin, dominates the strength level of the entire material, the resultant composite material has a high strength, as compared with the conventional reinforced plastic parts, as shown in FIG. 2.
The advanced composite material (ACM) forming method will be described in detail below.
Since thermoplastic resins, such as polyether etherketone (PEEK), exhibit high heat and shock resistance and can be repaired, their application to the aerospace field has been researched for a long time. However, since a sheet (prepreg) made of a carbon or glass fiber impregnated with a thermoplastic resin is hard at room temperatures, it is considered difficult to form such sheets laminated in a mold with a complicated shape.
On the other hand, a conventional prepreg made using a thermoset resin is soft and viscous and hence ensures easy forming. However, productivity of such a prepreg is not good.
Hence, to allow thermoplastic resins to be used in a composite material in place of thermoset resins, a forming method which is technically easy and reduces production costs has been desired.
As a means of achieving this objective, much attention has therefore been directed to co-woven fabrics of fiber of a thermoplastic resin and reinforcing glass or carbon fiber. Since such fabrics are flexible, they can be formed into a complex shape. Furthermore, the reinforcing fiber and the resin fiber are intermingled in these co-woven fabrics, they can be uniformly intermingled in a resultant composite material made by heating.
However, forming such co-woven fabrics are soft and hence cannot be handled easily. This makes stable high speed production difficult.
Furthermore, because the resin fiber must be melted again and be uniformly intermingled in the reinforcing fiber, impregnating it into the reinforcing fiber takes a long time.
Many high-speed metal forming methods have been developed over the years. Among the most common high-speed forming methods is sheet metal forming (one type of stamping), including deep draw forming. This forming method has much to offer with regard to mass production of resin type composite materials. Applying the concepts of this metal forming method to the forming of semi-finished products of sheets (prepregs) made of carbon or glass fiber impregnated with thermoplastic resin leads to a method which assures the highest productivity, because it eliminates the impregnation process required in forming of co-woven fabrics.
The semi-finished products which can be used in the above method, shown in FIGS. 3 (a) to 3 (c), are sheets in which oriented reinforcing fiber are impregnated with a thermoplastic resin.
In FIG. 3, (1), (2) .. indicate warps, and 12. . . indicate wefts.
These semi-finished products undergo any of the following deformations when they are formed by a combination of stamping and deep drawing: (1) fiber stretching, (2) fiber straightening, (3) shear slip and (4) caused by the trellis effect.
(1) Fiber stretching is deformation of fiber caused by tension acting on the fiber during formation, as shown in FIG. 4 (a). The maximum strain is as small as about 1%.
(2) Fiber straightening is deformation caused by loosening of warps and wefts of a fabric during forming, as shown in FIG. 4 (b).
To enhance the strength of a sheet, loosening of warps and wefts is controlled as little as possible during sheet weaving, and hence affects all of the deformations less.
(3) Shear slip is generated at a position where rapid deformation occurs, such as a corner, as shown in FIG. 4 (c).
(4) The deformation caused by the trellis effect is similar to the shear strain of metals, as shown in FIG. 4 (d), and is caused by changes in the direction of orientation of fiber, not by fiber straightening. A relatively small force produces a large deformation.
The aforementioned deformations have deformation limiting angles according to the type of fabric. When the sheet is deformed at an angle larger than this deformation limiting angle, local buckling occurs in the sheet, deteriorating the surface property of the finished product. During sheet forming, most portions of deformations are caused by the trellis effect shown in FIG. 4 (d).
Hence, it is necessary to control the trellis effect deforming angle within a predetermined angle during forming in order to eliminate local sheet buckling at that time and to obtain products having excellent properties (see PCT/EPC89/00428: WO89/10253).
In continuously manufacturing three-dimensional parts by heating and stamping from sheets of a fiber reinforced composite material which employs as a matrix a thermoplastic resin and in which the reinforcing fiber is oriented, the following factors must be overcome or improved.
(1) The sheet of the fiber reinforced composite material must be formed within the optimum temperature range determined by the type of composite material. Heating and forming the sheet at a temperature which is not within the optimum range (a) deteriorates the sheet flow during forming and thereby generates wrinkles or cracks in the finished products when the temperature is low; the temperature being maintained at Tg (glass transition temperature) or above during forming, and (b) deterioration occurs due to thermal decomposition of resin molecules when the temperature is high. In addition to these problems, color change may occur due to oxidation caused during heating.
This stamping method developed for mass production includes step 1: heating period; step 2: termination of heating and initiation of the forming (initiation of the mold contact); step 3: forming period (from initiation of the mold contact to termination of the mold operation; and step 4: cooling period. In these steps, the temperature of the sheet of the fiber reinforced composite material must be controlled as follows for the above-described reasons: (1) at the allowable heating temperature (for example, 210 .+-.10.degree. C. in the case of PA6 [polyamide: nylon]) or less when heating is terminated, (2) at Tg (glass transition temperature) or above (for example, 80.degree. C. in the case of PA6) when forming is terminated, (3) since the temperature of the sheet of the fiber reinforced composite material drops during steps 2 and 3, the forming device must be able to uniformly heat the sheet of the fiber reinforced composite material to the highest possible temperature within the allowable temperature range when heating is terminated.
In experiments conducted by the present inventors, when the sheet of the fiber reinforced composite material hangs by 40 mm during heating and the distance between the heater and the sheet of the fiber reinforced composite material thus becomes non-uniform, temperature difference of about 50.degree. C. occurs in the sheet. With 50.degree. C. temperature difference generated at the end of heating, the formable temperature range is narrowed, and the formable shape of products is thus limited.
Hence, a function of holding the sheet of the fiber reinforced composite material during heating is necessary in order to eliminate hanging.
(2) In the forming process according to the present invention, since an oriented semi-finished product (generally, V.gtoreq.50%) in which, unlike the conventional sheet thermal forming procecess, reinforcing fiber is woven in is used, generation of wrinkles in the finished product must be prevented by controlling the fiber placement during forming. When the ends of the oriented material are clamped and heated, the force for maintaining the relative distance between the warps and wefts in the woven fabric weakens and hence the relative distance changes slightly due to the weight of the semi-finished product, which causes it to hang. Generation of hanging deteriorates control of the fiber flow and hence generates the following problems. (a) When the hanging sheet is clamped by the upper and lower frames, wrinkles are generated in the clamped portions, deteriorating the surface property of the finished product. (b) Controllability deteriorates because play commensurate with the loosening of the fiber is generated.